1. Field of the Invention
The present invention relates to a copying apparatus with variable image magnification.
2. Description of the Prior Art
Recent progress in copier technology has provided multi-function copiers capable of copying in enlarged and/or reduced size in addition to the equal-size copying.
In such copiers, the image reduction or enlargement is conventionally achieved by displacing a lens and a mirror constituting the imaging system to predetermined positions according to a desired image magnification, thereby changing the dimension of the image projected on the photosensitive member.
Such lens and mirror are generally connected to each other through a mechanical link so that either one follows the movement of the other with a determined positional relationship therebetween.
In recent small copiers, however, the use of a drive mechanism involving such mechanical link between the lens and mirror is becoming spatially difficult. Also the moving spaces of the lens and mirror have to cross each other in certain cases and the conventional drive mechanism cannot cope with such requirement.
Such conventional copying apparatus is also provided with a light source, called blank exposure lamp, which is positioned close to the surface of the photosensitive drum and lighted while said drum is rotated but not exposed to the original image, thereby eliminating the charge in the non-copying area of the drum surface in order to avoid deposition of unnecessary toner on the drum.
This light source has to be turned off immediately before the scanning of the original document reaches the front end thereof, and is therefore controlled by time measurement from the start of said scanning. Stated differently said light source is turned on for a period corresponding to so-called pre-scanning distance from the start of original scanning motion.
In a copier with variable image magnification, in which the original scanning speed is generally variable, the measured time mentioned above should vary according to the scanning speed. In practice, however, said measured time is maintained constant for the purpose of simplifying the control program, and a white board is provided in a position to be illuminated by the pre-running of the optical system to dissipate the charge on the drum surface by the light of the original exposure lamp, thereby compensating for the aberration in the timing of blank exposure resulting from the difference in the image magnification.
However such compensation may not be effective enough since the light reflected from the white board can be weakened by the density adjust lever to be manipulated by the operator.
In high-speed copiers producing a large quantity of copies, the unnecessary loss of toner resulting from the above-mentioned incomplete charge elimination in the pre-scanning part of the optical system becomes large and cannot therefore be overlooked.
On the other hand, in copiers with variable image magnification, the copying in equal size and in varied size is achieved by varying the speed of the scanning system and the position of the optical system, and the scanning distance of the scanning system is limited to the necessary minimum in order to shorten the copy cycle time according to the selected image magnification. For this purpose switching elements such as microswitches are provided along the moving path of the scanning system, and one of said elements, selected according to the image magnification and copy size, is actuated by the scanning element to reverse the movement thereof. Also said switches are selected for controlling the scanning motion according to the sheet size in order to form the image within the size of the copying sheet.
There are however required a number of switches along the moving path as the number of image magnifications and of sheet sizes increases. Also in case the data for controlling the scanning distance are transmitted to and received from a microcomputer, the data lines of a limited number are almost occupied for this purpose.